#140 Render Screen Flashes w/Texture Selection Sidebar Visible

When in 'horizontal' and 'vertical' layout, when making changes to the mine, or moving around, etc. the mine screen will often flash from rendering, which appears to be the tools bars momentarily overtaking the mine screen until resized to where they are set to.

This does not happen in "vertical (space-saving)" because the texture selection sidebar isn't there, but also, when in that mode, there appears to be no method to bring up a texture selection window.

It makes those modes unusable, requiring "vertical (space-saving)" instead.